1. Field of the Invention
The invention relates to a power unit using a piezoelectric transformer, and an electrophotographic image forming apparatus and the like using the power unit.
2. Description of the Related Art
As a power unit used for electro-photographic image forming apparatuses, as described for example in Japanese Patent Application Publication No. 2006-91757, there has conventionally been known an apparatus which includes a piezoelectric transformer capable of producing a high voltage from a low voltage input by use of a resonance phenomenon of a piezoelectric vibrator, and which is configured to control the piezoelectric transformer by using an output signal of a voltage control oscillator (hereinafter, referred to as a “VCO”) so that the piezoelectric transformer outputs a high voltage.
However, the conventional power unit and an image forming apparatus using this power unit have problems (a) to (c) as below.
(a) The conventional power unit is formed of analog circuits such as a VCO. The piezoelectric transformers vary one to another in resonance frequency due to their production variation. Therefore, in the case of using a high output voltage in the vicinity of the resonance frequency in the piezoelectric transformer, a power unit needs to be equipped with components with different constants according to variation in the resonance frequency in order to correct the variation in the resonance frequency.
(b) In the case where the piezoelectric transformers having different sizes and different resonance frequencies are used, many of the constants of the VCOs and integrating circuits that form feedback circuits must be changed.
(c) There is a problem that the piezoelectric transformer becomes out of control when being controlled at a low frequency below the resonance frequency, and a high output voltage in the vicinity of the resonance frequency cannot be used.